Chức năng của túi giấy: Hướng dẫn toàn diện về các loại, chất liệu và ứng dụng trong công nghiệp

What Is a Paper Bag?

A paper bag looks simple. You see one every time you buy groceries, pick up takeout, or unwrap a delivery. But the brown kraft bag in your hand and the multi-wall industrial sack holding 25 kilograms of cement powder on a construction site share the same family name and almost nothing else in terms of what they can do.

At its core, a paper bag is a flexible container made from paper typically kraft paper derived from wood pulp engineered to carry, protect, and present its contents. The modern paper bag traces its industrial lineage to 1870, when Margaret E. Knight patented the first machine capable of mass-producing flat-bottomed paper bags, a design so enduring it sits in the Museum of Modern Art’s collection today (MoMA). What began as a hand-folded envelope has grown into a global industry worth an estimated USD 6.05 billion in 2024, projected to reach USD 7.47 billion by 2029 (MarketsandMarkets, 2025).

But treating “paper bag” as a single product category misses the point. The same fundamental technology cellulose fibers pressed into sheets, folded into three-dimensional shapes, sealed at the edges scales from a 5-gram bakery satchel to a 50-kilogram chemical-industry valve sack. Understanding paper bag function means understanding how materials, structure, and closure work together to create packaging that fits a specific job. That is what this guide unpacks.

The Core Functions of Paper Bags

Every paper bag function flows from three design principles: material determines baseline capability, structure determines spatial form, and closure determines the seal level. These three variables interact a weak point in any one compromises the whole. Get comfortable with this triad and you can evaluate any paper bag, whether you are buying a thousand retail bags or a container-load of industrial sacks.

Physical Protection and Load-Bearing

The most obvious function of a paper bag is also the most engineering-intensive: it must hold weight without failing.

A standard retail SOS (self-opening square) bag carries 2 to 5 kilograms without issue a few bottles, some produce, a hardcover book. Move to the industrial end of the spectrum, and a 4-ply valve sack routinely holds 25 to 50 kilograms of cement, fertilizer, or chemical powder. It survives drops from 1.2 meters onto concrete without rupturing, per the EN 27965-1 / ISO 7965/1 drop-test standards.

What makes this possible is the long-fiber structure of virgin kraft pulp. Softwood fibers primarily pine and spruce run 3 to 5 millimeters in length and interlock during papermaking to create a sheet with exceptional tear resistance. Hardwood fibers, at 1 to 2 millimeters, produce a smoother but weaker sheet. The result: an industrial paper sack’s strength-to-weight ratio can reach 1:250 one kilogram of paper safely containing 250 kilograms of product. No plastic film of equivalent weight matches this.

Fiber orientation matters as much as fiber length. Paper is anisotropic: it is stronger in the machine direction (MD, the direction fibers aligned during production) than in the cross direction (CD). Microcreping a process that introduces controlled wrinkles into the sheet can boost CD elongation from roughly 4% to 10%, giving the bag flexibility to absorb impact without bursting (Billerud Sack Handbook).

Containment, Preservation, and Barrier Functions

A bag that holds weight but lets its contents spoil is a failed bag. Containment means more than “not spilling” it means creating a controlled microenvironment around the product.

For dry goods like flour or cement, moisture is the primary threat. Untreated kraft paper absorbs ambient humidity readily; at relative humidity above 80%, paper bag strength can drop 30 to 50%. The fix is a barrier layer most commonly a polyethylene (PE) coating applied at 15 to 30 µm thickness. This reduces water vapor transmission rate (WVTR) from over 500 g/m²/24h for bare kraft to under 10 g/m²/24h. For products needing the highest protection coffee beans, milk powder, pharmaceutical ingredients an EVOH (ethylene vinyl alcohol) layer can be co-extruded, delivering an oxygen transmission rate below 1 cc/m²/24h.

Grease resistance is a separate challenge. Traditional solutions relied on PFAS-based coatings; the industry is now shifting toward fluorine-free greaseproof papers. These pass the 3M Kit Test at ratings of Kit 5 – 8 (moderate) to Kit 9 – 12 (high). They are essential for pet food bags, fast-food packaging, and any application where oil migration would stain the exterior or weaken the bag.

For the heaviest industrial applications chemical powders, carbon black, plastic resins the bag itself becomes a safety device. UN-certified dangerous-goods packaging (UN 5M1 for multi-wall paper, UN 5M2 for multi-wall water-resistant paper) must pass a battery of tests: 1.2-meter and 1.8-meter drops, 28-day stack compression, and sometimes a water-spray test. A bag that passes these is not just packaging; it is a regulated safety system.

Brand Communication and Information Display

The most overlooked function of a paper bag also delivers the highest return: it is a mobile billboard.

Flexographic printing on paper bags achieves 100 to 150 lines per inch (LPI) enough for crisp logos, product photography, and multi-color brand graphics. A well-designed bag turns every customer into a brand ambassador between the store and their destination, generating thousands of free impressions over its useful life. This is why premium retail brands invest in white kraft or clay-coated paper: the smoother surface produces sharper print, and the bleached white background communicates cleanliness and quality in a way brown kraft’s industrial aesthetic cannot.

But printing is not just marketing. For food-contact bags, the printed surface carries legally required information: ingredient lists, allergen warnings, nutritional panels, batch codes, and expiration dates. The inks themselves must comply with food-safety regulations the EuPIA exclusion list and Nestlé Guidance Note set limits of less than 100 parts per million for the sum of heavy metals (lead, cadmium, mercury, and hexavalent chromium) in food-contact printing inks.

Surface finish matters too. Brown kraft signals “natural, unbleached, eco-conscious.” White kraft signals “clean, premium, food-grade.” A matte finish feels understated; a gloss UV spot-coating on a logo adds tactile depth. These are not trivial aesthetic choices they shape consumer perception before the bag is even opened.

Paper Bag Types and Their Functional Differences

If the three design principles are material, structure, and closure, then bag type is where structure takes center stage. The six mainstream paper bag formats differ primarily in how their bottoms are formed, how they seal, and whether they include side gussets for expansion.

Beyond the table, one structural variable deserves extra attention: the gusset. A gusset is a folded-in side panel that lets the bag expand when filled and stand flat when empty. Bags without gussets flat satchels are the cheapest to produce but the most limited: they hold flat items well and bulky items poorly. Add side gussets to an SOS bag and its internal volume doubles or triples when opened, making it the default choice for grocery and retail. In industrial formats, gussets are engineered into the bag’s forming process and must match the filling spout on automated packing lines exactly a 2-millimeter mismatch can jam the line and halt production.

Valve-sack filling speed shows how structure and function intersect. High-porosity kraft paper (Gurley 3 – 5 seconds) lets the air displaced by incoming powder escape through the paper walls, removing the need for a separate deaeration step. That is why a cement valve sack fills in 3.5 seconds the paper breathes. A plastic bag would need a deaeration system, adding cost and complexity.

How Construction and Materials Shape Paper Bag Function

Understanding bag types tells you what is available. Understanding construction tells you why one bag works and another fails. Three engineering variables do the heavy lifting: ply count, material grade, and the barrier-plus-closure system. Upgrade any one without the others, and the weakest link still governs.

Ply Structure and Strength Engineering

A single sheet of 70 GSM kraft paper tears easily. Two sheets bonded together resist tearing far more than twice as well, because their fiber directions can be crossed. In a standard 3-ply industrial sack, the outer ply handles abrasion and printing, the middle ply provides bulk strength, and the inner ply often PE-coated delivers the barrier function.

The cross-ply effect is the quiet hero of paper bag engineering. When one ply’s MD (strong) fiber direction runs perpendicular to the adjacent ply’s MD direction, stress that would tear along a single-ply fiber boundary gets intercepted by the crossing fibers. A 4-ply bag rated for 25 kilograms does not simply have four times the strength of a single ply it has exponentially more tear-propagation resistance.

For procurement, the practical question is: how many plies does your product need? One to two plies handles lightweight dry goods under 10 kilograms with short, predictable supply chains. Three plies covers most bulk industrial products cement, fertilizer, animal feed up to 25 kilograms with standard transport handling. Four to six plies are for heavy or hazardous materials traveling long distances: export cement, bulk chemicals, mineral concentrates. In those cases, a bag failure at the bottom of a 20-pallet stack is not an inconvenience. It is a regulatory and financial incident.

Material Grades and Surface Properties

Not all kraft paper is equal. Virgin kraft made from freshly pulped softwood has the longest fibers, the highest tear resistance, and the highest cost. Recycled kraft uses post-consumer or post-industrial fiber; each recycling cycle shortens the fibers, reducing tear strength by 30 to 60% relative to virgin stock. Semi-bleached and fully bleached (white) krafts trade a small percentage of strength for appearance the bleaching process slightly degrades fiber integrity but the gap has narrowed with modern oxygen-delignification methods.

GSM (grams per square meter) is the standard weight metric. A 60 GSM sheet works for lightweight retail bags. An 80 – 90 GSM sheet serves standard grocery and takeaway applications. Industrial outer plies begin at 90 GSM and can exceed 120 GSM for heavy-duty sacks. But GSM alone is incomplete without tear index (mN·m²/g) and burst index (kPa·m²/g) two bags can share the same GSM and perform very differently depending on fiber source and papermaking process.

Surface properties count too. Calendered paper pressed between heated rollers produces a smooth, glossy surface ideal for high-quality flexo printing but with reduced friction. Uncalendered (“machine finish”) paper keeps a slight tooth that provides natural anti-slip grip. For palletized transport, friction is safety: a coefficient of friction (COF) below 0.5, measured per ISO 15359, means bags are likely to slide during truck transit. Anti-slip coatings or embossed patterns can boost COF without sacrificing print quality on the facing side.

Barrier Coatings and Closure Systems

The best ply structure is useless if moisture gets in or the closure fails. Coatings and closures operate as a paired system: the closure type determines the seal geometry, and the coating determines what that seal can resist.

PE extrusion coating (15 – 30 µm): The workhorse moisture barrier. Works for most dry goods; falls short for products needing oxygen or aroma barriers. Aluminum foil laminate: Near-zero moisture and oxygen transmission. The gold standard for coffee, dried milk, and pharmaceutical powders. EVOH co-extrusion: Transparent, high-oxygen-barrier (< 1 cc O‚/m²/24h). Used in premium food packaging. Water-based dispersion coatings: The emerging environmentally-preferred route recyclable or compostable without separation.

Closure choice follows the product’s physical form: heat-sealed pinch bottoms for fine powders (flour, starch, dried milk); machine-sewn closures with binding tape for agricultural and construction markets; self-sealing valves that eliminate closure as a separate step the internal valve leaf collapses under product weight the moment filling stops. No heat, no sewing, no adhesive just physics.

Paper Bag Functions by Industry Application

Theory matters. Application matters more. The paper bag functions that dominate in one industry may barely register in another. This chapter maps the four largest end-use sectors to their functional requirements, using the engineering framework built in the previous chapters.

Here is the selection logic, boiled down: identify your product’s primary demand on packaging speed (food), strength (construction), seal (chemicals), or presentation (retail) and work backward from that demand to bag type, ply count, and coating.

Bao bì thực phẩm và đồ uống

Food and beverage accounts for roughly half of all paper bag consumption globally. The functional priorities are a three-way balance: food safety, shelf-life preservation, and brand presentation with the first being non-negotiable.

For dry staples flour, sugar, rice, pasta a 2-ply pinch-bottom bag with a basic PE moisture barrier is the industry standard. Filling speeds of 15 to 25 bags per minute are typical on automated lines. The bag must survive warehouse stacking and retail shelf handling for 6 to 18 months.

Coffee packaging sits at the opposite extreme of functional demand. Freshly roasted beans emit CO‚ for days while being destroyed by oxygen. The solution a multi-layer bag with an aluminum foil or EVOH barrier plus a one-way degassing valve ranks among the most engineered consumer packages in existence. The result: a 12-month shelf life for a product that would stale in weeks without this functional architecture.

Construction and Building Materials

Cement packaging is where paper bag engineering reaches its purest industrial form. The functional challenge is direct: fill 25 kilograms of abrasive, hygroscopic powder in under four seconds, seal it without an extra step, stack it 40 bags high on a pallet, ship it across a continent, and lose zero bags to rupture.

The answer, refined over decades, is the high-porosity 3-ply valve sack. Porosity lets filling air escape through the paper walls at the same rate powder enters no air backup, no burst risk. The internal valve self-seals under product pressure. A modern cement packing line fills, seals, and ejects one 25-kilogram bag every 3.5 seconds over 1,000 bags per hour from a single spout.

Chemicals and Agricultural Products

Chemical and agricultural packaging sits at the intersection of every functional challenge at once: high weight, hazardous contents, long transport distances, and regulatory scrutiny. Take a fertilizer bag headed for export from a Chinese plant to a Brazilian farm. Three months at sea in a container that can hit 60°C and 95% relative humidity. The bag that survives this journey is not a commodity. It is a meticulously specified safety device.

For ammonium-nitrate-based fertilizers, the critical parameter is moisture absorption at the product’s critical relative humidity (CRH) about 59% RH. Below this, the fertilizer stays free-flowing. Above it and a Brazilian port regularly exceeds 80% RH the product cakes solid within hours unless the bag provides an effective moisture barrier.

Retail and E-Commerce Packaging

Retail and e-commerce paper bags run on a different functional logic. The product inside is rarely hazardous or heavy. The competitive battleground is the customer’s sensory experience in the first five seconds after receiving the package.

The functional triad for retail is print quality, handle integrity, and unboxing experience. Paper-twist handles must hold a static pull of at least 8 kilograms with a 2× safety factor. Handle failure in the customer’s hand is a brand disaster, far worse than a scuffed corner. E-commerce is speeding the shift from plastic to paper mailers paper mailing bags are growing at roughly 12% annually as brands respond to consumer backlash and regulatory pressure.

Paper Bags vs. Plastic Bags: A Functional Comparison

Choosing between paper and plastic packaging is an engineering decision, not a moral one. Each material has functional strengths the other cannot match. The right choice depends entirely on what your product needs starting from function, not from material preference.

The takeaway is not that one material wins. It is that paper and plastic are complementary technologies. A cement valve sack will never be plastic plastic cannot vent air during filling. A vacuum-sealed meat package will never be paper paper cannot hold a hermetic seal against liquids. The best packaging decisions start with function and let material follow.

To make this concrete: ask three questions before choosing. First, what is the worst thing that could happen to your product in transit moisture, impact, compression, or time? Second, does your filling line rely on porosity for speed? If yes, paper is your only option. Third, what does your local waste infrastructure actually process? A compostable bag sent to a landfill with no composting facility is just a more expensive landfill bag.

From Function to Production: How Paper Bags Are Made

After seven chapters of function, the natural next question is: how are these bags actually produced? The answer bridges the gap between understanding paper bag function and acting on that understanding whether that means placing a smarter procurement order or, for some readers, making the bags yourself.

• Step 1 Roll Stock: Kraft paper arrives from the mill in rolls weighing 1 to 2 tonnes. GSM, moisture content, and porosity are verified before production.
• Step 2 Printing: Flexographic printing on flat roll stock at speeds above 300 meters per minute, before the bag is formed.
• Step 3 Tube Formation: The tuber folds flat sheet into a continuous tube, applies adhesive along the longitudinal seam, and cuts into individual blanks.
• Step 4 Bottom Formation: The type-defining stage SOS, pinch-bottom, or valve-bottom machines each produce exactly one bag format.
• Step 5 Closure Preparation: Hotmelt pre-application for heat-seal bags; creasing for sewn bags; integrated with bottom formation for valve sacks.
• Step 6 Baling and Dispatch: Finished bags counted, compressed into bales of 250 1,000 units, palletized, and shipped.

Each step needs a specific machine and machine selection is where function meets investment. A fully automatic SOS paper bag machin a specific job. That is what this guide unpacks.e produces 200 to 400 bags per minute. A valve-bottom machine runs at 80 to 150 bags per minute the more complex bottom formation slows the cycle. The total investment for a production line capable of making multiple bag types at commercial volumes runs into the hundreds of thousands of dollars. That is why most bag users buy bags rather than make them.

For those considering the move into production, the key is matching machine capability to target bag type and volume. Equipment manufacturers serving this industry such as KETE, which offers customizable paper bag making machines spanning SOS, valve-bottom, pinch-bottom, and multi-wall formats with factory-direct pricing bridge the gap from functional specification to production reality. The same logic that guided bag selection in the previous chapters applies to equipment: identify your target bag type, production volume, and budget, then work backward to machine spec.

Tài liệu tham khảo

• MarketsandMarkets. “Paper Bags Market Global Forecast to 2029.” 2025. marketsandmarkets.com
• MoMA. “Flat-Bottomed Paper Bag.” Collection. moma.org
• Billerud. “Sack Kraft Paper Handbook.” billerud.com
• Britannica. “Margaret E. Knight.” britannica.com
• Southern Packaging. “Key Terms in the Multi-Wall Paper Bag Industry.” southernpackaginglp.com
• MarketsandMarkets. “Paper Bags Market worth $7.47 billion by 2029.” 2025. finance.yahoo.com
• Tập đoàn KETE. “Paper Bag Making Machines.” ketegroup.com